Finding inner peace is important for anybody, but even more critical at 40.
Growing ageism in the tech industry is a massive concern, however. But relearning the new thing every couple of years gets very old, so I completely understand how it happens.
Inner peace is the key - as you say - you have to give some missed goals the virtual Viking burial and let go of them. But I think you also need to set new goals or risk losing direction altogether.
Life is literally too short to care.
Because the superficial rule set of your surrounding social group dictates so?
If you don't harm other people and every now and then make someone happy or smile - then you did not, do not and will not fail!
Guys like Donald Trump are failures [1] - you most likely are not.
Do something to reshift perspectives if you're stuck in one that makes you feel so bad about your life that you even associate this with the subject at hand. Travelling, reading, moving, meditating, ...